How many vertices can I connect to a face?
If it's a big flat surface, it is absolutely fine to leave it as Ngons. A polygons with more than 4 vertices are called N-gons. For rendering and UV unwrapping there should not be much issue, if it's going to be staying within the blender environment.

However if you are working on models that would be exported into a game engine, it's best to define your 4 sided polys all over your meshes as much as possible. Games engine tends to mess up your triangles when no definition of edges are found. They pretty much auto tesselate your meshes for you, which usually produce "unhappy" results producing in-game artifacts.

The link that iKris posted is pretty clear on a number of reason when to use them, and when to avoid them.